126 



Dihybrids, Trihybrids, and Polyhybrids 



the limits of statistical error. This phenomenon, known as inde- 

 pendent assort77ient, is the second law of Mendel. It applies to 

 all ten pairs of chromosomes in maize and not merely to the two 

 pairs just mentioned. In maize, each of the ten pairs of chromo- 

 somes segregates entirely independently of any other pair, so 

 that gametes from Fi plants will be found with all numbers of 

 either maternal or paternal chromosomes from zero to ten. This 



Fig. 39. Four inherited types of rosette leaves in Capsella (Bursa). 

 These types result from the independent assortment of two pairs of genes. 

 Gene A draws out the primary lobes of the leaves into relatively long 

 sharp points whereas in aa plants the primary lobes are rounded. Gene B 

 divides the leaf by deep sinuses, usually reaching nearly or quite to the 

 midrib, but in hb plants the sinuses are not so deep. The four types from 

 left to right with their genotypes are: AB or heteris; Ab or tenuis; aB or 

 rhomhoidea ; and ab or simplex. (Photographs courtesy of Dr. G. H. Shull.) 



law of independent assortment applies to all plants and animals 

 as well as to maize (Fig. 39). 



This independent separation of the homologous chromosomes 

 of each pair was suspected on genetic grounds long before it was 

 demonstrated cytologically. It is normally difficult to show 

 cytologically because the two members of a pair of homologous 

 chromosomes are identical in appearance except in a few rare 

 instances. The independent segregation of two or more pairs of 

 chromosomes can be detected cytologically only if the two 

 homologues of each of the two or more pairs can be differen- 

 tiated from one another morphologically. Normally that is 

 impossible. Miss Carothers, however, found that in the orthop- 

 teran, Trimerotropis siifjusa, there are three pairs of homologues, 

 each consisting of one chromosome with a centromere at the end 



